My experience is that it’s expensive, building and contents £1600 and if you don’t have everything as specified like five yearly electrical inspection, the right type of locks on the doors etc, they are not very inclined to pay out in the event of a claim.
Personally I would spend the money on a good alarm system ( including a master blaster that makes it debilitating to be in the building ) that connects to a local central station who will come out in the event of an activation. Also make the place impregnable as much as you possibly can. It is extremely unlikely that they will be equipped with a lorry to take away your big bits of cast-iron, much more likely to be interested in power tools and handtools so lock these away every night in a very secure tool cupboard, they will not want to hang around trapped inside a building trying to get inside that cupboard, particularly whilst the alarm is going off and the master blaster is making it difficult to think. Ian
Edit also lock away your ear defenders.